Yeah you could definitely have a go.

Where is the exact line - i.e. can you use Lidar? Infrared depth-sensing? Or do these provide too much data such that the scene could be recreated?

(I'm exploring this as a thought experiment, in general I agree that people shouldn't be carrying hidden cameras on their faces, and if those cameras are at all connected to Meta then it's much worse!)

Well lidar in that form factor would end up just being an 8x8 laser DoF sensor like some smartphones have. There's no space or power budget for a real lidar.

That would be ok I guess. That's not enough to capture much of anything even with a continuous feed.